Climate models may underestimate future warming on tropical mountains

A new study that has reconstructed past temperature changes on Mount Kenya in East Africa, suggests that climate models may underestimate future temperature changes on tropical mountains.

‌An international research team featuring Swansea University Emeritus Professor Alayne Street-Perrott, has studied tropical mountains like Kilimanjaro and Mount Kenya, where the effects of climate change are particularly pronounced and the centuries-old glaciers have all but melted completely away. Now, new research, published in Science Advances suggests that future warming on these peaks could be even greater than climate models currently predict.

Professor Street-Perrott, of the College of Science, provided sediment samples from cores collected from two high-altitude lakes on Mt Kenya, Sacred Lake and Lake Rutundu, to the collaborative team, which was led by Dr James Russell, a Fellow at the Institute at Brown for Environment and Society.

From these samples, the study reconstructed temperatures over the past 25,000 years on Mount Kenya, Africa’s second-highest peak after Kilimanjaro. The work shows that as the world began to warm rapidly after the end of the last ice age around 18,000 years ago, average annual temperatures high on the mountain increased much more quickly than in surrounding areas closer to sea level. At an elevation of 3000 metres (around 10,000 feet), the average annual temperature rose 5.5 degrees Celsius from the ice age to the pre-industrial period about 150 years ago, the study discovered, compared to warming of only about 2 degrees at sea level during the same period.

The researchers found that when they ran state-of-the-art climate models over the same time period, these underestimated the temperature changes at high elevations, which implied that these models could also underestimate future warming on tropical mountains.

Temperature discrepancies

Discrepancies between temperature changes at sea level and at high altitudes have been the subject of debate for more than 30 years. Hence, it was thought that there must be something wrong with the geological evidence. More recently, novel geochemical methods have been developed by Dr Russell and his team to track temperature through time by studying the remains of ancient microbes extracted from lake sediments, in particular organic compounds known as GDGTs. These methods have proved accurate when tested on a large array of modern East African lakes.

‌The study presents new data from sediment cores taken from the bed of Lake Rutundu, an extinct volcanic crater lake on Mount Kenya, situated at an elevation of around 10,000 feet, which preserve the signature of GDGT chemistry dating back to the last ice age, more than 25,000 years ago. These cores have been carefully curated in cold storage at Swansea University.

The sediment analyses suggested that the average annual air temperature at Lake Rutundu has increased by about 5.5 degrees Celsius since the last ice age — a figure consistent with previous evidence from mountain glaciers and vegetation changes. However, the temperature reconstructions from two lakes closer to sea level — Lake Tanganyika and Lake Malawi — suggest much more modest temperature changes of about 3.3 degrees and 2 degrees, respectively.

The team found that while global climate models are able to reproduce the temperature changes at low altitudes, they underestimate high-elevation changes by 40 percent, which suggests there’s something amiss in the way the models simulate past changes in the atmospheric lapse rate, which is the rate at which air temperature decreases with altitude.

Dr Russell said: “All climate models calculate a lapse rate — it’s integral to the output of the model. What this work shows is that there’s a problem in the way the models make that calculation.”

Implications for the future

The research team says that while it is difficult to diagnose exactly what that problem is, it probably has something to do with the way models treat atmospheric water-vapour content. Water-vapour content is the strongest controlling factor governing the lapse rate, as moist air cools more slowly with altitude.

According to Emeritus Professor Alayne Street-Perrott, whose team took the original sediment-core samples from Mount Kenya, other potential issues with global climate models involve the simulation of cloud cover, cloud physical properties, and vertical exchanges of heat and radiation.

She said: “Whatever the source of the mismatch between the geological evidence and model simulations, the ramifications for tropical mountains may be significant. If the models miss almost half the temperature change at high elevations in the past, they may be underestimating future warming as well. Accelerated glacier melting may have serious implications for water resources and natural hazards in populated regions that are highly dependent on seasonal runoff from mountain ranges such as the Himalayas and the tropical Andes.”

Dr Russell said: “These are very fragile ecosystems that house extraordinary biodiversity and unique features such as tropical glaciers. Our results suggest that future warming in these environments could be more extreme than climate models currently predict.”